The sleep-wake cycle

• The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and animals should be asleep and awake.
• Light provides the primary input to this system, acting as the external cue for sleeping or waking. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN).
• The SCN then uses this information to coordinate the activity of the entire circadian system.
• Sleeping and wakefulness are not determined by the circadian rhythm alone, but also by homoeostasis.
• When an individual has been awake for a long time, homeostasis tells the body that there is a need for sleep because of energy consumption. This homeostatic drive for sleep increases throughout the day, reaching its maximum in the late evening, when most people fall asleep.
• The suprachiasmatic nucleus (SCN) is a bilateral structure located in the anterior part of the hypothalamus.

  It is the central pacemaker and regulates most circadian rhythms in the body
• Homeostasis is defined as a self-regulating process by which a living organism can maintain internal stability while adjusting to changing external conditions.